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Blues Junior Wrong Tube Installation & Cooked R32 resistor

Discussion in 'Amp Tech Center' started by zifir, Sep 6, 2020.

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  1. zifir

    zifir TDPRI Member

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    Hi there,

    Today I was doing my regular monthly-base maintenance to my old Blues Junior which has the green circuit board. Mistakenly, I installed the tubes in wrong direction. Therefore, two 12AX7 was put in place of the EL84s. As soon as I turned on the amp, I noticed a flame and smell of burn. After turning off, I saw that the R32 was cooked and turned to black.

    After a quick search on couple of gear pages, I learned that the R32 is voltage drop resistor (2.2Kohm, 2 watts) this issue occurs in case of a blown power tube or when it gets shorted.

    Actually, this is the only component shows a sign of physical damage in the amp and replacing the resistor is not a big issue. However, I am worrying about any other hidden issues. I am curious what parts might have been possibly damaged, or at least need to be checked.

    Any suggestion is welcome.
    Regards
     
  2. wabashslim

    wabashslim Friend of Leo's

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    Take it to a tech. From now on. For everything.
     
  3. Jon Snell

    Jon Snell Tele-Holic

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    Pin 9 of the EL84 socket goes through a 100R Fire Proof resistor R24 & R25, these are fed from the 2k2 R32 supply.
    Pin 9 of an ECC83/12AX7 is the centre tap of the heaters and will either have taken out the heaters or damaged a combination of the mentioned components.
    Replace R24, 25 & 32 with the correct type, throw the 12AX7s away.
    Fit the EL84s in the correct holes and a new pair of 12AX7s.
    Good to go again, if F1 did not fail. If F1 failed replace it with the exact type and value for your safety!

    R35 & 36 should have survived.
     
  4. Jon Snell

    Jon Snell Tele-Holic

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    If you have the Mk 3 version then the components are;
    R47, 35 & 36 respectively.
    R54 & 55 are the heater virtual earth resistors.
     
  5. zifir

    zifir TDPRI Member

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    Thanks a lot for the great info.

    Meanwhile, For R24 and R25, I couldn't find half watt metal oxide 100R
    Does it really matter if I replace them with carbon film type?

    Thanks
     
  6. Jon Snell

    Jon Snell Tele-Holic

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    Make sure they are fire proof!
    Carbon will burn and destroy your PCB when the screen grids fail in the EL84s.
     
  7. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Monthly??
     
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  8. tubegeek

    tubegeek Friend of Leo's

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    Yeah, you're supposed to shuffle the tubes monthly so they wear evenly. It's like rotating the tires on your car.
     
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  9. JRapp

    JRapp Tele-Holic

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    Wear evenly? Stop fussing with that---just play the thing.
     
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  10. wabashslim

    wabashslim Friend of Leo's

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    Take it to a tech. From now on. For everything.
     
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  11. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    I'm 99.9% sure this was a joke, but always hard to tell over the internet.
     
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  12. tubegeek

    tubegeek Friend of Leo's

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    Far be it from me to remove that last scintilla of mystery!
     
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  13. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    “100R” isn’t a term we should be using here at all.

    What are the listed resistor values, in *Ohms*, for R24 and 25, in the schematic you are looking at, for your exact model of amp?

    Example of a BJR (maybe not your exact model):

    B91FA1CA-CC2B-45C4-B4A2-5F075D218F00.png

    ALSO: Replacing resistors on a Blues Junior is a precision task, it is very easy to burn the circuit board and damage a trace. If you don’t have high grade tools to do the job, and if you haven’t practiced component replacement multiple times on junk pcb boards, please take it to an experienced repair person.

    ALSO2: Amp fixing is tricky in general, and if you aren’t super-careful and work in a very informed way you can easily destroy your amp and possibly get electrocuted.

    Good luck!

    PS: If you are passionate about becoming an amp tech, cool. Read verified texts for at least 1000 focused hours, and don’t assume the comments section in various random websites is accurate unless you *KNOW* that comment guy isn’t a guesser or a troll.
     
    Last edited: Sep 7, 2020
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  14. Dan_Pomykalski

    Dan_Pomykalski Tele-Meister

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    I normally assume anyone diving into an amp knows how to discharge caps and is aware there are lethal voltages stored in there. Sometimes the caps can store lethal voltages even after it’s been turned off. You are aware of this?
     
  15. tubegeek

    tubegeek Friend of Leo's

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    ?
     
  16. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    The replacement resistor values for either R24 or R25 in a BJR aren’t “100R”.

    If you disagree, please explain?
     
  17. tubegeek

    tubegeek Friend of Leo's

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    I thought you were complaining about the "100R" notation for a 100 ohm resistor. My mistake.
     
  18. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    Not complaining. Just sayin’:

    If anyone wants to call the R24 resistor in the schematic above an “820R” and can’t instantly find it online, ok... try searching for an “820Ω“ and you will find those on Amazon for 7 dollars for a hundred-pack, shipped.
     
  19. Nickfl

    Nickfl Friend of Leo's

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    Speaking of mystery, I still want to know what the monthly maintenance consisted of...
     
  20. dogmeat

    dogmeat Friend of Leo's

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    they don't have to be 1/2 watt... they can be more, like 1w, 2w, etc. as others have said though, not carbon. metal or ceramic.... "fire proof" means they burn less
     
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